Modulated microwave signals are used to carry information in a wide variety of electronic communications systems. Examples include modulated microwave signals used to transmit voice and data or video signals from a ground transmitter through space to a satellite, and then back from the satellite to a ground receiver. Another example is a television transmitter, which transmits modulated signals that carry the picture and sound to television sets. Another example is a cellular base station, which transmits modulated microwave signals that carry the voice information to cellular phones. Such signals must be accurately measured for conformance to systems specifications and for accurate modeling of deviations from ideal performance.
It is also desirable to use such modulated microwave signals to characterize nonlinear devices, such as power amplifiers, used in communications systems, because these are the signals that the devices receive in operation. Nonlinear electronic devices are the most difficult elements to model accurately in communications simulations. A recent example is the design and simulation of power amplifiers for use in digital cellular applications. In this case, the transmit power amplifier must be operated at or near saturation for high efficiency, and still meet stringent adjacent channel power requirements. This is an example where accurate, computationally efficient nonlinear models are required to make the proper design tradeoffs. Also known as black-box models, these models are computationally efficient because they transform an input waveform to the correct output waveform without resorting to the details of circuit operation. These models seek to characterize the nonlinear amplifier through the use of a selected set of probing signals. The degree of predictive fidelity of these simulation models must be checked with the class of operational signals expected, such as modulated microwave signals.
Accurate measurement of communication signals in the time domain may be used to construct and validate high fidelity communications system and component models. This is a significant advance over the traditional technique of basing such models on single-tone vector-network-analyzer measurements of the component or system. Unlike single-tone measurements, time-domain waveforms contain all the information necessary to accurately characterize the component or system over the bandwidth of the waveforms. In particular, nonlinear interactions between different frequency components of a communications signal are captured in a time-domain measurement of the waveform, but ignored in a traditional single-tone frequency measurement. Time-domain waveform measurements may be used to characterize nonlinear components such as power amplifiers. An outline of the procedure follows. A waveform source having the same modulation type as that intended for the application may be used as the test signal to be applied to the amplifier. Time-domain input and output measurements of these test signal waveforms capture the response of the power amplifier to the class of signals that are applied to it. In contrast, single-tone measurements cannot capture the nonlinear dynamic response of the amplifier to input signals having non-negligible bandwidth. A nonlinear model constructed from time-domain waveform measurements is accurate over wider bandwidths than models constructed from traditional single-tone input-output measurements.
The time-domain measurement of microwave communications signals is conveniently performed by first converting the microwave signal to baseband. This procedure yields inphase (I) and quadrature (Q) waveforms. The I and Q baseband waveform is commonly expressed in complex notation and is termed the lowpass equivalent (LPE) signal in the context of simulations. The accuracy of the measured waveform is limited by distortions introduced by the waveform measurement process. These distortions fall into four categories including linear filter distortions in the downconverting receiver, I and Q amplitude and phase imbalance, baseband nonlinearities such as amplifier compression and A/D non-ideality, and RF nonlinearities such as mixer compression. It is desirable to minimize the distortions. Limiting the input signal level to the downconverting receiver minimizes distortions due to baseband or RF nonlinearities. Linear filter distortions may be removed by a frequency-response calibration of the downconverter. The phase and amplitude response of the downconverting receiver cannot be measured directly by a vector network analyzer because the receiver input and output frequencies are different. The calibration procedure is used to extract the frequency response of the downconverting receiver by making three pair-wise measurements of two additional frequency converters and the downconverting receiver. One of the additional frequency converters must have a reciprocal frequency response, that is, the response must be the same whether it is used as an upconverter or a downconverter. This assumption of reciprocity is accurate enough to reduce linear filter distortions in the downconverting receiver to a low level over multi-Gigahertz bandwidths.
A prior procedure of measurement of modulated microwave signals is to record directly the radio frequency (RF) or microwave signal by means of a digital storage oscilloscope or other waveform recorder. For measurement of nonlinear devices, the RF or microwave frequency waveforms must be measured at the input and output of the device to find the input-to-output characteristic. These waveforms should be recorded at a number of input power levels throughout the operating range of the nonlinear device. Examples of such nonlinear components are a solid-state power amplifier or a traveling-wave tube amplifier. Time-domain instrumentation can record the waveform data digitally and store the data directly on a controlling computer.
Typical instruments used for recording waveform data are a digital storage oscilloscope (DSO), a microwave transition analyzer (MTA), or a recently developed large signal network analyzer (LSNA). The accuracy of these instruments for measuring high-frequency signals is limited by linear amplitude and phase distortion. The MTA and high bandwidth DSO have significant phase and amplitude distortion beginning at frequencies above about 15.0 GHz. The LSNA is based on an MTA, but the LSNA comes with calibration standards and an extensive calibration routine so that phase and amplitude distortion, and other errors, are analytically removed from the measurements. The LSNA performs calibrated time-domain measurements of signals up to 50.0 GHz. This LSNA includes calibration standards and software that calibrates the sampling oscilloscope for analog and digital nonlinearity, and gain and phase responses over a frequency range. Hence, the LSNA can provide accurate waveforms up to 50.0 GHz. The LSNA calibration eliminates any inaccuracy associated with the gain and phase response of the sampling oscilloscope, however, the LSNA still has limitations imposed by a limited number of samples and phase noise errors. The LSNA is an advanced and expensive system, however, and requires a specialized calibration procedure. Additionally, the LSNA can only be used to characterize arbitrary waveforms of a limited bandwidth.
Another prior waveform measurement approach is to use an uncalibrated downconverter with separate I and Q output signals. These I and Q output signals are then recorded by means of a DSO. This technique also yields the time-domain baseband waveform, but is corrupted by the linear filter distortions, and nonlinear baseband distortion, nonlinear RF distortions, and the I and Q imbalance between the I and Q signals in the downconverter. The unknown distortions can be large enough to severely limit the utility of the waveform data for a precision application such as communications system modeling.
A recent measurement approach measures the transmission response of a frequency-translating device (FTD), such as a mixer. The response of the FTD, including a downconverter, may be measured by means of the baseband-double-sideband-mixer FTD characterization method, as described in the related patent. In this FTD characterization method, three pair-wise combinations of an upconverter referred to herein as a transmitter, a test mixer, and the downconverter referred to herein as a receiver, are measured. The transmission response of the downconverting receiver is then calculated from these measurements. The test configuration setup for this FTD characterization method consists of connecting an upconverting transmitter FTD to a downconverting receiver FTD with both using the same local oscillator (LO) but with a phase shifter in the downconverter LO path. A vector network analyzer (VNA) is used to measure this first paired combination at two relative LO phase settings 90° apart. The additional test mixer is used in the second of these measurements as an upconverter and in the third of these measurements as a downconverter. The FTD characterization method requires that the test mixer have the same frequency response, which is a reciprocal frequency response, whether the test mixer is used as an upconverter or a downconverter. In practice, commonly available double-balanced mixers exhibit this reciprocal response if a low voltage standing wave ratio (VSWR) is provided on all ports by use of fixed attenuators. These six measurements, for the three configurations with zero and with ninety degree phase shift, are sufficient to extract the frequency response of all three FTDs, including the downconverting receiver. By mathematically combining the six measurements provided by the three-setup configuration, with and without the 90° phase shift, the lowpass equivalent (LPE) frequency response of the downconverting receiver may be obtained.
In U.S. Pat. No. 6,211,663 entitled Baseband Time-Domain Waveform Measurement Method, issued Apr. 3, 2001, a time-domain baseband measurement method measures modulated microwave signals typically used in communication systems by converting microwave signals to baseband before measurement for improved accuracy compared to direct measurement at the microwave frequency. A downconverting receiver is first characterized using a prior characterization method and then the modulated microwave signal is applied to the downconverting receiver and the response of the downconverting receiver is removed to provide an accurate characterization of the modulated microwave signal. Such an accurate measurement of the modulated microwave signal can be used for communications system performance verification as well as for characterizing communications devices and systems. One particular application is the measurement of input/output characteristics of nonlinear power amplifiers using such modulated microwave signals. Such a system produces imbalances of the I and Q signals upon carrier demodulation. The method includes inserting local oscillator phase shifts for measuring downconverter DC offsets. However, the method does not remove all I and Q imbalances that lead to distortion of the input signal during downconversion. These and other disadvantages are solved or reduced using the invention.